A diary of the TanDEM-X formation swapping

Between 6 and 9 August 2013, and after great preparation, the team of the TanDEM-X mission was excited and ready for the swap formation of the twin satellites TanDEM-X and TerraSAR-X.

Swap Day 1 - Tuesday 6 August 2013:

The tension is building up in the TerraSAR-X and TanDEM-X control room. The team has gathered to monitor and conduct the activities for the satellite swap. In three days, the orbit of TanDEM-X around TerraSAR-X will be reset, while TerraSAR-X will remain unchanged. The objective is for TanDEM-X to circle TerraSAR-X clockwise (in the direction off flight) and not, as in past years, in an anti-clockwise direction. So the duo will be able to observe target areas on Earth's surface from the opposite viewing angle, which is necessary when measuring difficult areas such as mountains.

After several weeks of preparation, the time has come. Exciting days lie ahead. At this moment, the two satellites remain in close formation at a distance of only about 280 metres. To perform the swap safely, it is necessary to increase their separation. On 6 August 2013 at 08:24:16 UTC, manoeuvres by TanDEM-X to cause it to fall back relative to TerraSAR-X begin. The team is ready and the increasing distance between the two will soon become apparent. Normally, TanDEM-X receives telemetry data from TerraSAR-X – this will become increasingly difficult as the distance between them grows. How long will communication still be possible? TanDEM-X has to be prepared by the team in the control room for the unusual silence of its partner. A global network of ground stations from Germany, Canada, Antarctica and Spitsbergen is available for communication between the control centre and the satellite. During an overflight, they each allow a contact time of about 10 minutes. It takes about one and a half hours for the two satellites to orbit Earth. When planning the swap, an attempt was made to provide one contact per orbit in this phase. Indeed, is a whole series of tasks to be performed.

On the first day, the orientation of TanDEM-X is slightly modified to reflect the changes in its relative geometry with respect to TerraSAR-X that will be accomplished over the next three days. After the swap, TanDEM-X will again be able to observe the same ground track on Earth's surface as TerraSAR-X.

Swap Day 2 - Wednesday 7 August 2013:

TerraSAR-X and TanDEM-X have now drifted to a safe distance from each other, with 10 kilometres of separation in the direction of flight. Now it is time to perform the crucial manoeuvres with TanDEM-X to lay the foundations for the new formation. This requires that, on 7 August 2013 at 07:09:08 UTC, TanDEM-X is rotated 180 degrees so that its thrusters act in the desired direction. Each rotation takes about eight minutes; the actual manoeuvres in between take approximately 70 seconds. The team can also follow much of the action live in the control room, as TanDEM-X is in its visibility window over the O'Higgins ground station in the Antarctica. After this, TanDEM-X begins to catch up with TerraSAR-X. Meanwhile, attention is needed in the control room – TerraSAR-X will also have to be prepared for the new formation. The team has to adapt to changes in the two satellites. Today, the definition of protective measures on the two satellites is on the programme. These ensure that the partner satellite is unable to be illuminated by radar transmissions. There is also a maintenance phase, in which the nominal radar image acquisition by the TerraSAR-X and TanDEM-X missions is paused to provide an opportunity to perform activities that cannot be readily performed during the radar operation. This is used, among other things, to make special measurements on the oscillators in the radar instruments.

Swap Day 3 - Thursday 8 August 2013:

Today it is quiet. Most of the preparatory activities have been completed. But the new formation is not in place! TanDEM-X is still drifting towards TerraSAR-X. The braking manoeuvre to stop the relative drift with respect to TerraSAR-X is pending. The orbit of TanDEM-X must be raised slightly. Just one small correction and the new formation is ready. Only the final steps remain for the restarting of the radar mission early the next morning. Done!

Image 1: Subsystems and flight dynamics engineers of AOCS (Attitude and Orbit Control Systems) observe the execution of a manoeuvre, DLR/Edith Maurer, CC-BY.

Image 2: The German Space Operations Center (GSOC). This is the control room during the formation swap of TerraSAR-X and TanDEM-X, DLR/Edith Maurer, CC-BY.